1. Field of the Invention
The present invention relates to a control method for a direct current (DC) brushless motor, and particularly, relates to a pulse width modulation (PWM) control method for a sensorless DC brushless motor.
2. Descriptions of the Related Art
Currently, DC brushless motors typically employ a pulse width modulated (PWM) power input to change speeds. However, such a PWM input signal has an impact on the back electromotive force (BEMF) detecting circuit, causing an incorrect BEMF signal to be generated by the BEMF detecting circuit. Consequently, when a zero crossing (ZC) occurs to the incorrect BEMF signal, a mistaken phase switching may take place and cause a failure in the normal operations. Accordingly, it has been important to prevent mistaken phase switching.
U.S. Pat. No. 5,767,654 discloses a method for detecting a BEMF. According to this method, the time at which the BEMF crosses zero is predicted, and a PWM input signal is maintained high prior to the zero-crossing. Normal PWM operations occur only when the BEMF detecting circuit detects the zero crossing of the BEMF.
U.S. Pat. No. 5,789,895 discloses another method for detecting a BEMF. According to this method, a reference value is preset. Once the BEMF crosses this reference value, a PWM input signal will be maintained high. Normal PWM operations resume when the BEMF detecting circuit detects the zero crossing of the BEMF.
However, both methods provide detection of a correct BEMF signal, but at a cost of suspending normal PWM operations. Consequently, the DC brushless motor still cannot operate according to the normal PWM signal at all times, causing failure of the motor to operate at a steady speed.
In view of this, it is highly desirable in the art to provide a control method and a circuit for preventing incorrect detections of a BEMF while maintaining a normal operation of the motor.